Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

RASA 11 - problems with reflections/rainbows caused by bright stars

  • Please log in to reply
49 replies to this topic

#1 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 30 December 2021 - 03:29 PM

I use a RASA 11 as my primary imaging rig, and since switching to a full frame ASI6200MC camera and the Celestron Light Pollution Imaging Filter for the RASA 11, I get a lot of rainbows in all of my images, most likely caused by internal reflections from bright stars. The stacked image below shows a few examples of this and I've highlighted the most obvious reflections.

 

Although the rainbows make my images look nice and colourful, I want to get rid of them! I'm looking to get some help and advice from the community if possible, as I struggle to fix these issues in processing.

 

Does anyone have any recommendations of how to fix these reflections - either in processing (e.g. in PixInsight), or by adjusting my imaging rig slightly (this would be the preferred option)? At the moment I just tend to use the "clone" tool in Lightroom or Photoshop to paint over the rainbows, but some images are difficult to fix due to the surrounding nebulosity.

 

Also, it's worth noting I use the Baader UFC series of adapters for attaching the camera to the RASA, and I believe these adapters aren't the cause of the issue as they have a matte finish on the inside to mitigate reflections.

 

Thanks in advance!

Attached Thumbnails

  • horsehead-reflections.jpeg

Edited by danielgallo, 30 December 2021 - 03:34 PM.

  • th3r3ds0x likes this

#2 bugbit

bugbit

    Vendor - Octopi-Astro

  • *****
  • Vendors
  • Posts: 476
  • Joined: 28 Apr 2015
  • Loc: west of seattle

Posted 30 December 2021 - 06:30 PM

There is a member here that had issues sort of similar to what you are experiencing that goes by the name Fegato. I don't know if he ever got it resolved but a PM to him may get you pointed in the right direction.


  • danielgallo likes this

#3 freestar8n

freestar8n

    MetaGuide

  • *****
  • Freeware Developers
  • Posts: 13,309
  • Joined: 12 Oct 2007
  • Loc: Melbourne, Australia

Posted 30 December 2021 - 09:08 PM

I think in this case they may be caused by Mintaka, which is just out of view at the top - though it's hard to be sure.

 

I would remove the light pollution filter because it is likely not helping much - and use a long dew cap if possible.  Are you using any kind of shield in front?  If the reflections are due to a bright star out of the field - it may help.

 

And examine the optical path near the camera for anything smooth that might cause a reflection - even if it looks black.  Use flocking on any smooth surfaces.

 

But if it is ultimately caused by the internals of the RASA corrective lens system, then you may not be able to fix it.

 

Frank


  • danielgallo likes this

#4 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 31 December 2021 - 07:12 AM

There is a member here that had issues sort of similar to what you are experiencing that goes by the name Fegato. I don't know if he ever got it resolved but a PM to him may get you pointed in the right direction.

Thanks, I'll try and find his posts and see if he found a solution!

 

I think in this case they may be caused by Mintaka, which is just out of view at the top - though it's hard to be sure.

 

I would remove the light pollution filter because it is likely not helping much - and use a long dew cap if possible.  Are you using any kind of shield in front?  If the reflections are due to a bright star out of the field - it may help.

 

And examine the optical path near the camera for anything smooth that might cause a reflection - even if it looks black.  Use flocking on any smooth surfaces.

 

But if it is ultimately caused by the internals of the RASA corrective lens system, then you may not be able to fix it.

 

Frank

Thanks. I use an Astrozap aluminium dew shield. I'll take a closer look at the optical path for any smooth/reflective surfaces.

Is flocking just a case of getting some matte black non-reflective paint to cover over any potentially reflective surfaces?



#5 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 31 December 2021 - 12:50 PM

Hello - just spotted this....

 

yes you have exactly what I was suffering from - rainbows of all sizes caused by stars outside field of view, and sometimes inside it. I have Baader UFC from scope to camera, and was convinced it wasn't this...

 

But to cut a long story short - the problem is either reflections from the Baader image train, or the thread inside the RASA lens group that the Celestron filter / optical window screws into (when you use the Baader UFC, this window is obviously removed).

 

I was back and forth with FLO for weeks trying this and that, as I suspected a RASA lens group problem. Finally I got a piece of flocking paper (you can buy rolls from FLO), cut it to the length of the whole image path (i.e. from inside that RASA thread up to the filter holder), rolled it up inside (just stuck the edge to itself - you can't stick it properly inside the Baader adapters), and the problem has completely gone.

 

I haven't narrowed it down to the Baader or the RASA thread, partly because I just don't want to take it apart again right now! I should add that I had tried black matt paint inside the Baader adapters which made no difference. 


  • bugbit, danielgallo and RaulTheRat like this

#6 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 31 December 2021 - 04:04 PM

Thanks Robin, I appreciate the reply!

 

I'll go ahead and order some of that flocking material from First Light Optics, and hopefully that'll sort it for me also.



#7 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 02 May 2022 - 06:48 AM

Thanks Robin, I appreciate the reply!

 

I'll go ahead and order some of that flocking material from First Light Optics, and hopefully that'll sort it for me also.

Hi Dan

 

Did this sort out your reflection issue?  

 

I want to try and sort out a slightly more permanent fix for this than having flocking paper floating around, and having added a PP adapter at the front to avoid that awful clamp ring, it makes it difficult to know how well the paper is positioned in there (my camera is on an Octopi device, so removing that to check is not an option without mucking everything up). I don't think the paper will stick to with its own glue, but was wondering whether to risk sticking it down with something stronger, maybe just at the lens group end where I think the problem occurs.

 

Was just wondering if you had any experience to share before I do anything...?  thanks!



#8 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 02 May 2022 - 04:06 PM

Hi Robin!

 

Yes, the flocking material did sort the issue - I rolled a piece of it up inside the Baader adapters, from the front of the camera through to the light pollution imaging filter on the RASA (thanks for your suggestion!). That appeared to solve the issue.

 

I've also just recently switched to the Octopi Astro adapter though, and the issue has returned so I need to add the flocking again. It's a bit more difficult with the Octopi adapter though. I've gone for the entire Octopi setup, and done away with my various Baader adapters and Precise Parts RASA adapter. I was wondering whether to try cutting out a single hollowed-out circular piece of felt to stick around the edge of my light pollution filter, rather than rolling up a piece inside the adapter as it doesn't stick easily - I haven't had a clear night to test yet though.

 

I've heard a few more reports of this issue elsewhere (a few people have reached out to me via my website asking if I fixed it). I'm sure the issue is somewhere in or around the lens group area - maybe a shiny edge of a lens or metal piece that's causing a reflection/refraction of bright stars. Did you happen to report the issue to Celestron at all? I'm wondering whether they have investigated this, as it's obviously not an isolated issue.

 

Thanks!


Edited by danielgallo, 02 May 2022 - 04:08 PM.


#9 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 03 May 2022 - 02:11 AM

Thanks for the update!  

 

Well, I'm not certain to be honest. When I got the PP adapter, I thought I'd take the glass out of the Celestron optical window and put the frame into the lens group. However, the reflections returned. I'd taken the flocking out completely, so can't be sure where it came from.

 

If we had plenty of clear nights to play with, I'd do more experiments, but there we are. 

 

I think I will bite the bullet and stick some flocking in the front of the RASA using gorilla glue. I'll leave the Baader train, and see what happens with that.



#10 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 03 May 2022 - 04:10 AM

OK forget sticking, bad idea, nothing wants to stick on those threads. I've cut a small piece at the full diameter of the lense group to just fit as tightly as I can get it into the front there (left the backing paper on and stuck with sellotape!). I'll try this and see what I get. 

 

NB I noticed that the narrower piece I had along the whole Baader tube has ended up leaving some sticky residue on the glass on the front of the lens group, despite leaving the backing on! I guess the full Octopi adapter is the full width of the lens group? So you could have a single piece at full width down to the filter, which I think would work better.

 

Why we buy these expensive bits of kit and then have to fiddle about with bits of paper and sellotape I don't know!...



#11 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 13 May 2022 - 11:38 AM

I use a RASA 11 as my primary imaging rig, and since switching to a full frame ASI6200MC camera and the Celestron Light Pollution Imaging Filter for the RASA 11,

I hadn't grasped that you were using the Celestron filter - is that still the case with the Octopi?

 

In my latest test, I took the frame of the Celestron filter with everything else removed, and used it to hold in place flocking back up to the lens group glass. I'm suspicious that the reflections are coming from both of the smooth black surface inside the screw-in filter holder that holds the glass in place (now all removed), and the smooth black surface behind it right in front of the lens group glass.

 

However - with that in place I still get reflections. But these are less rainbow-like and more separated. Possibly just the front edge of the Celestron filter holder? I don't know whether I can get flocking to stick on that, but I'll have a go.

 

So far the only thing that has stopped this is rolled up flocking all the way to the lens group, and with the Baader UFC this is narrower than the lens group glass - so (a) I'm worried it is reducing the light cone coming in, and (b) it leaves sticky residue on the glass, which ain't great!



#12 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 13 May 2022 - 05:00 PM

Hi Robin,

 

It's interesting you mention adding some flocking material behind the screw-in filter - I did something similar earlier this week and had a chance to capture some test images a few nights ago.

I essentially cut out a circular piece of flocking material to fit snug in the lens group and then cut out another circle within that leaving about a 9mm-10mm border of flocking material around the edge (see photos below - is this what you also tried?). I left the backing paper on the flocking material. I then held the flocking material in place behind the Celestron Light Pollution Filter - I use this most of the time as I'm using a colour camera. 

 

Based on the test images of M101, I'm not noticing any rainbow artifacts after stacking the data (the rainbows were normally very faint on single frames, but more obvious on stacked images). See the attached stacked image with a quick DBE in PixInsight.

 

Now maybe I need to pick a different target, but this one has quite a good amount of bright stars around the edges, so I would have thought this would've caused some rainbow artifacts? I guess to prove this fixes the issue I need to remove the flocking material and perform the same test on the same target - I'll try that another night.

Attached Thumbnails

  • IMG_3031.jpg
  • IMG_3032.jpg
  • IMG_3033.jpg
  • PixInsight.jpg


#13 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 14 May 2022 - 04:21 AM

Ah yes, that's interesting. No, I was still using it as a small tube behind the filter holder, just protecting the sides in front of the lens group glass - as here:

 

lens group filter flock small.jpg

 

The latest reflections I have might be coming from that front edged marked in red. But this filter holder wasn't there at all earlier, so previously must have been from somewhere else (I suspect where the flocking now is). There also seem to be plenty of bright surfaces inside the lens group itself, but not much I can do about that (apart from blocking light path as you have done).

 

So anyway, thanks, that gives me something else to try as well! I can see how it would prevent stray light at the edges hitting anything just in front of the lens group there, or block strays from behind. I'm sure the problems are coming from this area. My only worry with this is that we'll be blocking the designed light path, but I guess that has to be judged in the final images. Did you see any noticeable vignetting (or more extreme vignetting if you had it already) or any other issues?

 

I'll definitely get on to Celestron if we can tie this down, if only so that they know, as I don't suppose they can do anything for us really. Does the full Octopi set up have a wider tube than the lens group all the way to the camera? If so - it really can only be the Celestron end that is causing this.



#14 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 14 May 2022 - 05:40 AM

Not very pretty, but I'll give this a go! If it fails, the reflections must be from inside the lens group, so your approach will be the only option.

 

lens group filter double flock small.jpg

 

(and yes, I need to clean that mark off the lens group glass!)



#15 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 24 May 2022 - 06:18 PM

Hi Robin,

 

It looks like the flocking material around the edge of the lens group does fix the issue - at least with my limited testing this evening. I decided to remove the flocking material and imaged a patch of the sky that exhibited rainbows, then re-added the flocking material and imaged the same exact patch of sky to see if the rainbows disappeared.

 

In the screenshot below, you can see on the left two faint rainbow artifacts on the single 60-second frame captured with no flocking material. Then on the right, the same exact framing and camera rotation, but this time with the flocking material. These are just single 60-second frames with an ABE in PixInsight. There are no rainbows visible!

 

Rainbows.jpg

 

I actually added a narrower circle of flocking material this time (~5.5mm), and that appears to be enough. I made sure it was black on both sides by sticking two bits of flocking material back to back, just in case the white backing paper causes any side effects due to its reflectivity. Obviously time will tell whether this definitely resolves the issue, or whether any other bright stars in different parts of the field of view still cause rainbows. 

 

FlockingMaterial.jpg

 

Have you had a chance to test something similar?


Edited by danielgallo, 24 May 2022 - 06:24 PM.


#16 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 25 May 2022 - 01:34 AM

Thanks, Daniel, that's good to know. I haven't had a chance yet, but we've got clear nights forecast over the weekend, so hopefully will do so shortly.

 

Should be interesting. I think if I still get them, it probably means the reflections are coming from inside the lens group itself. Your little circle probably blocks any stray rays from just inside the glass there - it certainly is a shiny surface. I think I'll prepare a circle like yours so I can try that too if necessary.



#17 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 28 May 2022 - 02:30 AM

Well, that didn't work. Massive reflection from Deneb when shooting Sh2-115. Tonight I'll try your little ring - it was all prepared, but I couldn't face pulling things apart at midnight. If it can block a reflection from Deneb, it will block anything!



#18 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 29 May 2022 - 05:34 AM

Yes, the little 5mm ring does the job - thanks for coming up with that. Even with Deneb so close, no reflections, just a bit of glare which is probably inevitable with such a bright star so close.

 

So there we are. There's no doubt in my mind that the reflection is coming from inside the lens group - this ring will be blocking them. I think all other possibilities have been exhausted with the various different bits of flocking. When I get a bit of spare time, I'll put together a dossier and send it to Celestron. Just for their info really, not sure they'll do anything.



#19 danielgallo

danielgallo

    Sputnik

  • -----
  • topic starter
  • Posts: 49
  • Joined: 03 May 2018
  • Loc: Andover, Hampshire

Posted 30 May 2022 - 04:07 PM

Yes, the little 5mm ring does the job - thanks for coming up with that. Even with Deneb so close, no reflections, just a bit of glare which is probably inevitable with such a bright star so close.

 

So there we are. There's no doubt in my mind that the reflection is coming from inside the lens group - this ring will be blocking them. I think all other possibilities have been exhausted with the various different bits of flocking. When I get a bit of spare time, I'll put together a dossier and send it to Celestron. Just for their info really, not sure they'll do anything.

Excellent, I'm glad to hear it helped!



#20 PabloGazmuri

PabloGazmuri

    Lift Off

  • -----
  • Posts: 3
  • Joined: 02 Aug 2022
  • Loc: Medfield, MA

Posted 04 August 2022 - 02:07 PM

Hey Folks,

 

I ran into this very same issue converting to Baader UFC system. I didn't have a good way to stick flocking into the right place, and I didn't want to fuss with cutting to precision, so I created a 3d printed part to do the job:

 

https://www.thingive...m/thing:5447235

 

I tested this last night and can confirm the reflections are gone, and I see no vignetting on my APS-C sized sensor.

 

Hopefully this option will be helpful to others.


  • Fegato likes this

#21 Fegato

Fegato

    Viking 1

  • -----
  • Posts: 566
  • Joined: 15 Nov 2018
  • Loc: Llanfynydd, Carmarthen, Wales

Posted 04 August 2022 - 02:26 PM

Hey Folks,

 

I ran into this very same issue converting to Baader UFC system. I didn't have a good way to stick flocking into the right place, and I didn't want to fuss with cutting to precision, so I created a 3d printed part to do the job:

 

https://www.thingive...m/thing:5447235

 

I tested this last night and can confirm the reflections are gone, and I see no vignetting on my APS-C sized sensor.

 

Hopefully this option will be helpful to others.

excellent - thanks, I'll get one printed up. A bit of floppy flocking paper is not ideal! 

 

I'm amazed more people haven't come across this



#22 calypsob

calypsob

    Cosmos

  • *****
  • Posts: 8,935
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 04 August 2022 - 05:30 PM

Interesting solutions, anodized black reflects infrared like nothing is there at all. Slap an ir pass over 600nm and point at anything black anodized, it shows up silver. To truly attenuate light you need to paint the surface black, flock it, mask it via 3d baffle. Thats the only way to stop it. The aluminum around the sensor can also reflect light back onto the corrector. Its crazy that manufacturers of all quality levels overlook this critical detail 

 

here is a great writeup. With this in mind, the RASA is a minefield of ir reflective surfaces.

 

https://www.photo-sp...ised-aluminium/


Edited by calypsob, 04 August 2022 - 05:35 PM.


#23 calypsob

calypsob

    Cosmos

  • *****
  • Posts: 8,935
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 04 August 2022 - 05:36 PM

excellent - thanks, I'll get one printed up. A bit of floppy flocking paper is not ideal! 

 

I'm amazed more people haven't come across this

Petg accepts acrylic nicely btw



#24 PabloGazmuri

PabloGazmuri

    Lift Off

  • -----
  • Posts: 3
  • Joined: 02 Aug 2022
  • Loc: Medfield, MA

Posted 04 August 2022 - 08:32 PM

Yeah, I painted mine with rust-oleum high heat black for grills as (I'm told) it does not reflect in IR/UV... stuck well to the ABS+ that I used to print the piece.


  • calypsob likes this

#25 jcrop

jcrop

    Lift Off

  • -----
  • Posts: 21
  • Joined: 17 Aug 2020
  • Loc: Texas!

Posted 21 October 2022 - 09:25 AM

I've recently ran into this issue as well.  I've had my Octopi attached to my RASA 11 v2 for about a year.  I run the rig with an ASI6200MC camera as well.  

 

Not until about 2 weeks ago when I got serious about leveling the tilt of my camera via the Octopi did this 'rainbow glare' issue begin.  Before I got all four corners of the tilt adapter within 3-5 microns of each other was this 'effect' an issue.  My stars evened out with adjusting the tilt, but this glare effect started.  Ugh.  haha

 

I can get the flocking paper easily, but I'm concerned with cutting it accurately.  How did you guys cut it with precision?

 

 

PS. If anyone would care to sell me a 3D printed ring I'd be interested, but the one posted here is for an APS-C sensor and not a full frame, so I don't know if it'd even work.  I do not own a 3D printer.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics